As the demand for diesel boiling range fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing diesel fuel. One such source is what has been termed biorenewable sources. These biorenewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of glycerides and Free Fatty Acids (FFA). Both of these classes of compounds contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic chains in the glycerides or FFAs can be fully saturated or mono, di or poly-unsaturated.
There are reports in the art disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Applicants have developed a process which comprises an optional pretreatment step, and one or more steps to hydrogenate, decarboxylate, decarbonylate, (and/or hydrodeoxygenate) and optionally hydroisomerize the feedstock, and which can be successfully operated at a lower pressure range than previous systems. Employing a volume ratio of recycle hydrocarbon to feedstock ranging from about 2:1 to about 8:1 provides a mechanism to increase the hydrogen solubility in the reaction mixture sufficiently so that the operating pressure of the process may be lowered. The range of successful volume ratios of recycle to feedstock is based upon the desired hydrogen solubility in the reaction mixture. The reaction zone may be operated at a pressure in the range of about 1379 kPa absolute (200 psia) to about 4826 kPa absolute (700 psia).